Clock or timing recovery circuits have the purpose of extracting a timing wave at the symbol rate from a conditioned signal pulse stream. Once recovered, such a timing wave might then be used, for example, to clock subsequent signal processing stages for the pulse stream. The present invention is particularly concerned with the case where the original data stream is in the form of a modulated optical signal, as might be found, for example, in an optical telecommunications system. Hitherto, in order to recover a clock signal from such an optical data stream the optical signal has been converted from the optical to the electronic domain using e.g. a photodetector, and the electronic signal subsequently filtered to derive the timing wave. Since the bandwidth of the electronic circuitry is inevitably far smaller than that of the optical circuits, this clock recovery stage acts as a bottle-neck, limiting the performance of the system of which it forms a part.